Mold coating composition,particularly for centrifugal molds



U.S. CI. l0638.27 Claims ABSTRACT OF THE DISCLOSURE An elongated centrifugal casting mold and a refractory United States Patent 0 "ice coating is provided for casting thin walled tubular members wherein the coating is made up by weight of 60% to 97% of one of the group consisting of pulverulent zircon and lamp black, 1% to diatomaceous earth, 2% t0 The present invention relates to mold coatings for use in preparing molds for casting metal and the like, and

- more particularly to a refractory mold coating for metal molds use in centrifugally casting iron, steel, high alloy steels, and other metals.

When making long castings of molten metal, and par= ticularly when castings relatively long tubular members by centrifugal techniques, the art has long been confronted with the problem of producing a casting in which the metal is completely fused throughout the body of the casting, and which has a smooth, substantially defect-free outer surface. 1

These problems are more or less acute depending upon the particular method of casting employed, and in any case an acceptable solution thereto has not hitherto been found.

When the molten metal is poured into the mold, a too rapid cooling of the outer surface thereof frequently re sults in pin holes and in similar surface defects, partic ularly in the case of metal molds. In the case of centrif= ugal molds or in other molds having considerable length or intricate passages, chills laps, or cold shuts are pro duced when the leading edge of the molten metal must sometimes extend throughout the thickness of the body of the casting, but usually occur only in the outer portion of the wall, which is sometimes referred to as the outer surface of the casting.

Attempts, therefore, to produce long tubular castings centrifugally or other lengthly intricate castings in un coated stationary molds have hitherto resulted in the for= mation of castings having the aforementioned pin holes and laps or cold shuts which render the castings commercially unacceptable.

In other molding techniques, the mold was coated with an intentionally roughened coating to slow the advance of the molten metal charge moving lengthwise of the mold. It was conjectured that the aforementioned pin holes and other defects have resulted from a too rapid advance of the molten metal through the mold thereby giving rise to such defects. In the case of centrifugal cast ing techniques, it was further conjectured that the provision of a rough mold coating would hasten the mold pick-up of the molten metal charge so that the molten 3,447,936 Patented June 3, 1969 metal would quickly partake of the rotational motion of the centrifugal mold to afford a more uniform casting. Thus, it was proposed to provide the mold surfaces with a Coating having a large number of protuberances, which would hasten the molten metal pick-up and/or slow the lateral advance of the molten metal throughout the mold, This mtehod was successful to some extent in reducing the pin holes and laps or cold shuts referred to previously, but has had a severe limiting effect upon the length of the tubular casting or other casting made in accordance with this conventional technique. For most applications, more= over, the surface of the centrifugal casting was commercially unacceptable due to the rough outer surfaces thereof produced by the roughened mold coating.

In a more recently developed method, a resin-bonded sand is coated upon the mold surface for the purpose of slowing down the cooling rate of the molten metal cast therein. The lateral advance of the molten metal throughout the mold also is slowed down in this method due to the roughness of the sand coating. Although this method involves some improvement over prior methods, the resultant surfaces of the castings still are not sufficiently smooth for many commercial applications. The restricted lateral movement of the molten metal, by the resinbonded sand coating, also limits the length of the castings which can be fabricated by this technique, particularly in the case of centrifugal castings.

The coating compositions disclosed herein overcome these problems by providing a mold coating which permits rapid transfer of the molten metal charge to the extremities of the mold before the molten metal has an opportunity to develop localized congealing and-the aforementioned pin holes and laps or cold shuts. In the case of centrifugal casting, the mold therefore, when coated with the disclosed coating compositions, permits rapid lateral transfer of the molten metal along the longitudinal axis of the mold. The outer surface of the casting, moreover, is free from defects, is smooth, and does not require further finishing. These aims are accomplished by the disclosed coating compositions, which, when properly applied to mold surfaces, produce an exceptionally smooth refractory coating thereon to promote rapid advance of the molten metal over the mold surfaces and to provide an exceptionally smooth surface of the finished casting. The coating is composed from suitable refractory materials which will not decompose at elevated temperatures with attendant oxidation of adjacent molten metal or with undesirable release of gas. The refractory coating of the invention, moreover, introduces a controllable thermal impedance, depending upon the particular composition of the coating, which provides a uniform thermal insulation for the molten metal at the mold surfaces to prevent localized congealing or chills in the molten metal surface. In many applications, the casting can be utilized without further surface treatment, owing to the smoothness of the mold coating produced from the disclosed composition. Where an extremely smooth surface finish of the casting is required, the coating after application to the mold surfaces is sufficiently adherent to admit of further finishing prior to use of the mold, by conventional techniques such as sanding.

In the case of centrifugal castings, the disclosed coating composition makes possible the production of tubular castings made of steel or other metals or alloys, of a length not hitherto possible without exhibiting the aforementioned pin holes, or laps or cold shuts, as would detract from the quality of the castings or render them commercially unacceptable. The centrifugal castings thus produced meet present-day standards of surface finishes with-= out further grinding, polishing, or other smoothing operation.

The coating composition disclosed herein facilitates centrifugal casting techniques by promoting the distribution of the molten metal poured into the mold in both the circumferential and longitudinal direction. The rate of pickup of the molten metal by the mold can be accelerated by more rapid rotation thereof. At the same time, the longitudinal distribution of the molten metal is accelerated by the provision of a very smooth mold coating resulting from the use of the disclosed coating compositions. Slower rotation also facilitates longitudinal distribution. The heat flow to the mold from the leading edge of the longitudinal ly advancing molten metal is sufiiciently arrested by the coating composition to maintain leading edge of the tem perature to fuse the leading edge portion with molten metal which subsequently flows thereover. This is accomplished by-providing a coating composition which can be sprayed in sufiicient thickness to provide the required thermal insulation, and which contains one or more com ponents which inherently enhances the thermal impedance of the coating.

The aforementioned coating composition can be applied for example, by suitable and appropriate coating techniques, as by spraying a liquid suspension of the coating composition upon the mold surfaces, and the aforementioned required properties of the coating can be varied and controlled by proper selection of spraying equipment or the like. For example, various arrangements for apply ing the coating composition to centrifugal molds and the like are described and claimed in applicants copending and co-assigned application filed concurrently herewith entitled Manufacture of Tubular Metal Castings, Ser. No. 493,477.

Compositions which have been found suitable for providing a mold coating having the desired characteristics referred to above are com-posed of a coating suspension based primarily on zircon flour (pulverulent zirconia) or able in an atomizing type of spray equipment, and are desirably of about No. 325 seive size or finer. These com= ponents, particularly the zirconia, have fusion points at elevated temperatures and thus enable the molten metal to be poured into the mold at hotter temperatures, with the result that a casting substantially free of defects can be obtained. The lamp black base coating composition is particularly useful for the centrifugal casting of molten cast iron. Moreover, both of these components are advantageously used at many applications because of their non= toxicity. The bentonite is added to maintain the coating composition in suspension in its liquid carrier and when coated upon the mold surfaces aids in promoting the ad herence of the coating to the mold surface.

The diatomaceous earth, when utilized, accomplishes three functions: it increases the thermal impedance of the mold coating to prevent the development of localized chills in the poured molten metal charge, it allows easy stripping or removal of the coating from the mold fol lowing the casting operation, and it acts as an inert binder when the coating is applied to the mold. The sodium silicate, when added, also aids in suspending the coating composition in its carrier and, in addition, promotes the bonding of the coating to the mold surface and thereby prevents the coating from spalling in certain applications.

EXAMPLE I A coating having the following composition by weight has been found to yield excellent results when applied to mold surfaces for centrifugally casting steel or the like.

Percent Zircon flour 87 Diatomaceous earth 10 Bentonite as 3 Water, sufficient for 60 Baum density.

Depending upon the particular coating procedure adopted, a density of the resulting suspension can vary between about 50 and degrees Baum. If the suspension is too fluid, such as under 50 Baum, it will settle too rapidly during the coating procedure; on the other hand,

if the Baum density is too high, the mixture becomes too thick to apply properly and uniformly. Suitable carrier liquids other than water can be employed as a carrier for the suspension, depending upon the application of the invention. In the case of centrifugal casting techniques, the aforementioned coating composition facilitates the casting of longer tubes than was heretofore feasible. Moreover, a casting is obtained which is free from defects and has an excellent surface finish. The coating composition affords the necessary thermal impedance for castings of this length, and because of the refractor nature of its components, permits the molten metal charge to be poured into the mold at a much hotter temperature. Since mold coatings are expendable, particularly in the case of centhat stripping is diflicult, removal of the coating can be facilitated by increasing the proportion of diatomaceous earth. The proportion of diatomaceous earth can be increased for this purpose up to about 25% by weight or about 60% by volume. An increase in diatomaceous earth also furnishes the mold coating with additional thermal impedance where required.

In those few cases where the coating is too friable before casting or where it is otherwise desired to increase the bonding action of the coating suspension to the mold, a quantity of sodium silicate can be added of up to about 4% by weight. To increase the suspendability of the coating composition during spraying or other coating techniques, the bentonite component can be increased up to about 20% by weight. In the higher ranges of bentonite, and particularly in those applications where a high thermal impedance is not required, it is possible to reduce the amount of diatomaceous earth component substantially.

EXAMPLE II 5 Accordingly, for various mold coating applications, the tollowmg ranges by weight of the coating composition disclosed herein have been found to be effective:

Water to provide a density of 50 to 80 degrees Baum.

EXAMPLE III Mold coatings produced from the following ranges of coating composition also have been found effective in many applications for the purposes described above:

8 Percent by volume Zuconia 40-60 Diatomaceous earth a- 40-60 Bentonite b. 2-8

Water for 50-70 degrees Baum density.

In the last-mentioned example, up to 4% by weight of sodium silicate can be added to increase the bonding effects of the coating to the mold surface or to prevent spalling of the coating where such occurs.

EXAMPLE IV Percent Lamp black, No. 8792 Diatomaceous earth 5-10 Bentonite r. 3 Sodium silicate 0-4 Water to provide a 50 degree Baum density.

This coating composition also produces the desirable mold surface and thermal impedance characteristics of the previous examples, with the coating base, lamp black, being substituted for the zirconia or zircon flour.

In the coating compositions of the latter three examples, the components bentoni'te and diatomaceous earth, and sodium silicate where used, perform essentially the same functions as described above, in these respective compositions.

From the foregoing it will be apparent that novel and efficient coating compositions have been disclosed herein. While there have been shown and described certain presently preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims. For example, it is abundantly clear that new compounds and new uses of known compounds are being discovered almost daily and that one or more of such compounds can be readily substituted for one or more components of the disclosed coating composition in order to carry out the combination of functions discovered and disclosed herein.

1 claim:

1. In combination an elongated centrifugal casting mold and refractory coating on the interior working surface of said mold for casting tubular members, said coating comprising by weight 97% of one of the group consisting of pulverulent zircon and lamp black, 125% diatomaceous earth, 220% bentonite, and 04% sodium silicate;

2. In combination an elongated centrifugal casting mold and refractory coating on the interior working surface of said mold for casting tubular members, said coating comprising 40-60% by volume of pulverulent zircon, 40-60% by volume of diatomaceous earth, and 2-8% by weight of bentonite.

3. In combination as elongated centrifugal casting mold and refractory coating on the interior working surface of said mold for casting tubular members, said coating comprisingby weight about 87% of pulverulent zircon, about 10% of diatomaceous earth, and about 3% of bentonite.

4. In combination an elongated centrifugal casting mold and refractory coating on the interior working surface of said mold for casting tubular members, said coating comprising by weight about 87% of pulverulent zircon, about 10% of diatomaceous earth, about 2% of bentonite, and 0-4% of sodium silicate.

, 5. In combination an elongated centrifugal casting mold and refractory coating on the interior working surfaces of said mold for casting tubular members, said coating comprising by weight 87-92% of lamp black, 5-10% of diatomaceous earth, about 3% bentonite, and 0-4% sodium silicate:

References Cited UNITED STATES PATENTS 425,346 4/1890 Royal et a1 106-3825 2,282,349 5/ 1942 Wellings et al 106-3828 2,399,606 4/ 1946 Schul et a1. 106-3827 2,623,809 12/1952 Myers 106-3828 3,056,692 10/ 1962 Kitada 117-53 3,164,871 1/1965 Haughton 117-53 X 3,230,056 1/1966 Arant et a1 106-38 X 3,275,460 9/1966 Jeanneret 106-3822 JULIUS FROME, Primary Examiner, L. HAYES, Assistant Examiner.

US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,447,936 June 3 1969 Martin N. Ornitz It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 28, "castings" should read casting line 42, "chills" should read chills, Column 2, line 6, "mold" should read mold. line 7, "mtehod" should read method Column 3, line 32, "493,477"

should read 493,747 Column 4, line 74, 2-8" should read 2-8 Percent. by

weight Column 6, line 4, "220% should read 2-20% line 32, "Schul" should read Schuh Signed and sealed this 14th day of April 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Officer 

